Experimental Analysis and Modeling of Closed-Loop Redox Flow Desalination

Abstract

Redox flow desalination (RFD) is an emerging electrochemical process utilizing a redox couple for salt separation, which enables continuous and energy-efficient brackish water desalination. Several redox couples have been used to desalinate water along with energy production (from sustainable sources such as sunlight) and storage. However, an in-depth analysis of the redox reactions and ion transport that occur simultaneously in different components of an RFD electrochemical cell is still lacking. We carried out a series of experiments, with different cell configurations, to elucidate the contribution of individual components to the cell voltage. A closed-loop RFD mediated by ferri-/ferrocyanide as a model redox couple was designed for this analysis. The cell voltage gradually increased as the diluate concentration decreased from 3 to 0.5 g l−1, attributed to the solution resistance, membrane resistance, Donnan potential, and overpotential of the redox reaction. We experimentally measured voltage drops due to each component, the sum of which was in good agreement with the time-variant cell voltage. A phenomenological model was used to fit the experimental concentration and voltage data, and a good fit was obtained. The combined experimental and modeling analysis reported herein is expected to guide a rational design and operation of RFD processes.